This invention relates to austenitic stainless steel alloys which have improved resistance to both thermal creep and swelling when exposed to nuclear radiation. The alloys comprising the invention are basically nickel-chromium steel alloys which have closely controlled additions of minor alloying constituents. These minor alloying constituents, in the proper quantities, provide the resulting alloys with improved resistance to helium embrittlement and improved resistance to void swelling during irradiation, as well as thermal creep resistance.
The invention is a response to a continuing need for improved steel alloys for use in both radiation and high temperature environments. This need is particulary apparent in the area of nuclear fission or fusion reactors, as the intensely radioactive environment is extremely damaging to existing steel alloys. In particular, neutron irradiation of steel alloys used, for instance, as fuel element claddings or structural members, induces transmutation reactions which lead to the production of impurities such as helium. Although helium is an inert gas, it is highly insoluble in steel alloys and tends to form bubbles along the grain structure of the alloys. Furthermore, the presence of interstitial helium, and the damage caused by the irradiating neutrons, produce dimensional changes in the steel alloy, manifested as physical swelling, which has serious deleterious effects on the mechanical properties of the steel alloy and which may lead to failure. The results of irradiation, including embrittlement (loss of ductility) and swelling, inevitably shorten the useful life of the steel components, thereby having a significant negative economic impact on the nuclear power and research industries.
The damaging e ffects of helium embrittlement and swelling on the integrity of steel alloy reactor components are well known. In the prior art, efforts have been made to modify existing steel alloys by either changes in composition or by special thermomechanical treatment during fabrication. See, e.g., Bloom et al., U.S. Pat. No. 4,011,133, and Bloom et al., U.S. Pat. No. 4,158,606. In particular, some measure of success in swelling resistance has been achieved by increasing the concentrations of silicon and titanium in conventional austenitic stainless steel alloys. However, these alloys show only slightly greater resistance to radiation-induced embrittlement at elevated temperatures than do existing alloys such as type 316 stainless steel. This is because earlier efforts were aimed primarily at the problem of radiation-induced swelling alone without regard to the problem of helium embrittlement, which is a function not only of helium build up along grain boundaries, but also of grain boundary carbide distribution in the alloys. Some measure of success in reducing helium embrittlement during irradiation has been achieved by heat aging a titanium modified austenitic stainless steel to produce MC carbide along the grain boundaries prior to irradiation. See Maziasz & Braski, 141-143 J. Nucl. Mat'ls--(to be published in 1987). Consequently, there remains a need for improved steel alloys which offer greater resistance to both radiation-induced swelling and embrittlement than existing alloys.